When forming images with coherent radiation, the desired distribution of image energy is subject to undesirable random modulation. This random distribution of energy is known as speckle and is manifested in visual images as flecks of random intensity and size distributed across the image. Speckle arises from constructive and destructive interference due to random phase cancellations and additions of the coherent field scattered by the coherently illuminated object. The power spectrum of speckle depends upon the spectrum of the coherent signal carrier, the texture or spatial distribution of scatterers in the field, the size of the irradiated object volume, and the transfer function of the receiving and imaging system.
One frequency-diversity technique to suppress speckle in coherent optical images was first demonstrated and reported in "A Wavelength Diversity Technique for Reduction of Speckle Size," by M. Elbaum, M. Greenebaum, and M. King, Optics Communications, Volume 5, Number 3, pp. 171-174, June, 1972, which describes the use of multiple frequencies to reduce speckle in optical systems. In this method, the intensity distributions of images obtained by transmitting narrrow-band coherent illumination at different frequencies are superimposed (added) noncoherently. For this method to be effective, the spectrum of the illuminating energy must be selected to assure that object-scattered signals at component illuminating frequencies are decorrelated by virtue of object texture. Such conditions are described in "SNR in Photocounting Images of Rough Objects in Partially Coherent Light," by M. Elbaum and P. Diament, Applied Optics, Volume 15, Number 9, pp. 2268-2275, September 1976.
In ultrasonic diagnostic or materials-inspection systems, a broadband, coherent pulse of ultrasonic energy is directed into an object being examined and is scattered upon transmission through the object. Scattered energy is then detected coherently to produce a voltage signal having a spectrum equivalent to the irradiating signal spectrum, but altered by the effects of attenuation, scattering, and interference phenomena. Irradiating pulses can be transmitted at different orientations (e.g., with angular, or linear displacements of the illuminating beam between pulses) or at different times. Scattered signals can be processed (envelope detected) to form video signals from which an image can be generated. Because these images are generated from coherent echo signals, they are subject to speckle degradation as are coherent images produced by coherent light.
Four prior approaches for reducing speckle effects in ultrasound signals have been demonstrated. All suffer from (1) degradation of system figures of merit such as resolution, (2) complexity of transmission and scanning methods and apparatus, or (3) extended acquisition time.
One prior approach used in ultrasonic scanning systems generates reduced-speckle images by directing ultrasonic bursts or pulses from several different directions, then adding the obtained images noncoherently or superimposing them. A second prior approach involves noncoherently adding sequentially obtained images. However, both of these approaches suffer disadvantages. The method using pulses from several directions assumes that refraction does not preclude proper alignment of component images. The method using sequential pulses assumes that no major tissue motion occurs during the examination period. Neither of the foregoing assumptions are necessarily true, and particularly in medical systems, serious resolution degradation can result either from motion or acoustic refraction.
A third prior approach applies low-pass filtering to the video signal, which in effect blurs the image, and reduces the distracting effect of speckle having a high spatial frequency. However, this approach also degrades resolution.
A fourth prior approach sequentially transmits ultrasound in different frequency bands. The separate images formed from echoes in each of these bands are added, which supresses speckle in the resulting image because the speckle in each constituent image is not correlated. However, a separate transmission is required for each frequency band, which increases the time of examination and requires a complicated transmission system. In medical imaging and other applications, the risk of image degradation due to motion increases as examination time increases.